The present invention relates broadly to devices for directing a fluid stream across a rotary junction and, more particularly, to a device for directing at least two fluid streams, including a relatively high pressure water stream and a relatively low pressure slurry of air and abrasive particles, across a rotary junction while maintaining the compositional integrity of the slurry.
Rotary couplers are typically used in situations requiring a fluid stream directed from a central source to be moved through a predetermined angular displacement for aiming. Typically, these couplers should be rotatable through greater than 360.degree. without reversing direction. Accomplishment of this task is a simple matter if one fluid stream is involved. For example, in Tuson et al U.S. Pat. No. 4,250,918, a fluid stream is directed along the rotational axis of the rotary coupler. This method positions the body of the rotary coupler radially outwardly from the fluid stream so that, essentially, the coupler rotates about the fluid stream.
Problems arise, however, when separate, multiple fluid streams are passed across the rotary junction. In that situation, the additional fluid streams must be positioned radially outwardly from the axis of rotation with generally cross-sectionally washer-like channels formed in the coupler, as seen in Grace, U.S. Pat. No. 4,214,780. The solution employed by Grace works when the fluid stream is a homogenous liquid capable of experiencing turbulent flow. However, the solution is insufficient for use in the field of water jet cutting. There, a mixture of abrasive particles in water is directed to a nozzle to be selectively aimed at a work piece, for example, a turbine blade, for removing worn portions thereof. In this case, there are typically two flow streams involved, to be mixed at the nozzle. One is a high pressure water stream on the order of 50,000 psi. The other is a slurry of abrasive particles, usually garnet, and air. The air/garnet slurry moves under the influence of a slight vacuum. Just prior to reaching the cutting orifice, the water stream flows through a venturi tube. Inside the venturi tube, a partial vacuum is created to draw the abrasive slurry into the water stream. This partial vacuum causes the slurry to travel through supply lines toward the mixing nozzle. Mixing must occur immediately prior to cutting application because the mixed stream is so abrasive that only a carbide nozzle can withstand the abrasive stream. Cost requirements prevent lining a coupler with a carbide material and, therefore, mixing is confined to the nozzle.
In order to properly position the water jet nozzle for cutting, a rotary coupler is provided which allows 360.degree. movement of the nozzle. Due to the aforesaid abrasiveness of the mixed cutting stream, the abrasive slurry and the water jet must be passed through the rotary head separately. In the past, this has been accomplished by passing the water jet though a conduit formed along the rotational axis of the rotary coupler and a separate, flexible tube directing the abrasive slurry to the mixing nozzle. However, this separate, flexible tube prevented free rotary motion of the nozzle due to its tendency to wrap around the rotary coupler and the nozzle as the nozzle was rotated beyond a certain angular displacement. Accordingly, to move the nozzle beyond a certain angular displacement, the direction of rotation would have to be reversed, resulting in unnecessary movement and perhaps damage to the flexible abrasive tube. To compound the problem, the slurry is at such a low pressure that if the conventional method of multiple fluid line rotary couplers were employed, the disruption in flow of the slurry as it entered the necessary cylindrical chambers would cause the abrasive garnet to drop out of the slurry stream and the steam would tend to pulse, resulting in improper mixing at the mixing nozzle, causing uneven water jet cutting. Therefore, the slurry must remain substantially undisturbed and intact through its passage through the rotary coupler. Turbulence, direction reversal or blockage of the slurry can cause the garnet to drop out of the mixture or otherwise cause an uneven mixture of air and garnet which causes the aforesaid pulsing at the mixing nozzle. Therefore, the need exists for a rotary coupler which can successfully pass therethrough a high pressure water stream and a low pressure abrasive slurry.